In the field of water cooled nuclear reactors it is well known that the most probable accident in the nuclear reactor is a reactor core meltdown which occurs when the capability to remove heat from the nuclear reactor core is lost. When a reactor core meltdown occurs, the core melts through the pressure vessel and the molten material drops onto the containment floor, which is typically fabricated from a concrete material. The reactor is surrounded by a containment building which is intended to prevent the radioactivity from escaping into the environment.
The resultant mixture of molten and solid core material from a water cooled nuclear reactor core melt accident is "corium". Corium results from a zirconium clad fuel and is generally comprised of uranium oxide (UO.sub.2), steel (Fe), zirconium oxide (ZrO.sub.2), zirconium (Zr) and fission products. In a typical coremelt accident, the molten corium core is an uncoolable, high power density mass which exits the bottom of the reactor pressure vessel. The geometry of the molten corium is ill defined. The high temperature corium can penetrate the containment floor by high temperature decomposition of the concrete. When this occurs, the reaction between corium and the concrete generates flammable, noncondensable gases. More specifically, the decomposition of concrete generates carbon dioxide (CO.sub.2). The resultant carbon dioxide reacts with the zirconium in corium to generate carbon monoxide (CO). Further, zirconium reacts with water in the containment and in the concrete to generate hydrogen (H.sub.2). The noncondensable gases, hydrogen and carbon monoxide, may overpressurize the reactor containment causing its failure. Also, the hydrogen and carbon monoxide may burn or detonate with air in the containment building which could cause containment failure. Furthermore, with failure of the containment, the potential for leakage of radioactive material to the environment is extremely high.
Many methods for preventing containment failure have been proposed. Generally, these methods require spreading the molten material out so it can cool and freeze by flooding the underreactor cavity with water. The basic drawback to all such approaches is ensuring the distribution or geometry of the molten core
material during a core-melt accident. For example, lead core catchers have been developed in an effort to solve the problems involved with a core-melt accident. Lead has a sufficiently high density to float the core debris but there is no way to absorb the core debris such that the power density of the molten core is lowered. Further, a lead core catcher can not prevent reaction of zirconium in corium with the water which drains down onto the corium with time.
A core-melt source reduction system has been developed for a gas cooled fast reactor which utilizes a stainless steel clad fuel. The major difference between the system developed for the stainless steel clad fuel and the present invention for a zirconium clad fuel is that stainless steel does not react with most materials in a core-melt accident. Unlike stainless steel, zirconium is highly reactive. Because of this highly reactive nature, there will be fundamental compositional differences between the system developed for the stainless steel clad fuel and that for zirconium clad fuel.
Therefore, it is an object of this invention to provide a core-melt source reduction system which stops the progression of a high temperature core through the containment floor during a core-melt accident.
It is another object of the present invention to provide a core-melt source reduction system which prevents the generation of noncondensable gases when the core materials react with the containment floor.
It is a further object of the present invention to provide a core-melt source reduction system which does not require any assumptions about the geometry or timing of the molten core material in a core-melt accident.
It is yet another object of the present invention to provide such a core-melt source reduction system which can be incorporated into or replace the existing containment floor.
Further, it is object of the present invention to provide a core-melt source reduction system which minimizes heat rejection to the containment floor early in the core-melt accident.
It is yet another object of the present invention to provide a core-melt source reduction system which ends the accident sequence with a long term, cold, stable state.
It is a further object of the present invention to provide a core-melt source reduction system which traps radionuclides in a solidified matrix.